//
// low-level driver routines for 16550a UART.
//

#include "types.h"
#include "param.h"
#include "memlayout.h"
#include "riscv.h"
#include "spinlock.h"
#include "proc.h"
#include "defs.h"

// the UART control registers are memory-mapped
// at address UART0. this macro returns the
// address of one of the registers.
#define Reg(reg) ((volatile unsigned char *)(UART0 + reg))

// the UART control registers.
// some have different meanings for
// read vs write.
// see http://byterunner.com/16550.html
#define RHR 0			// receive holding register (for input bytes)
#define THR 0			// transmit holding register (for output bytes)
#define IER 1			// interrupt enable register
#define IER_RX_ENABLE (1<<0)
#define IER_TX_ENABLE (1<<1)
#define FCR 2			// FIFO control register
#define FCR_FIFO_ENABLE (1<<0)
#define FCR_FIFO_CLEAR (3<<1)	// clear the content of the two FIFOs
#define ISR 2			// interrupt status register
#define LCR 3			// line control register
#define LCR_EIGHT_BITS (3<<0)
#define LCR_BAUD_LATCH (1<<7)	// special mode to set baud rate
#define LSR 5			// line status register
#define LSR_RX_READY (1<<0)	// input is waiting to be read from RHR
#define LSR_TX_IDLE (1<<5)	// THR can accept another character to send

#define ReadReg(reg) (*(Reg(reg)))
#define WriteReg(reg, v) (*(Reg(reg)) = (v))

// the transmit output buffer.
struct spinlock uart_tx_lock;
#define UART_TX_BUF_SIZE 32
char uart_tx_buf[UART_TX_BUF_SIZE];
uint64 uart_tx_w;		// write next to uart_tx_buf[uart_tx_w % UART_TX_BUF_SIZE]
uint64 uart_tx_r;		// read next from uart_tx_buf[uart_tx_r % UART_TX_BUF_SIZE]

extern volatile int panicked;	// from printf.c

void uartstart();

void uartinit(void)
{
	// disable interrupts.
	WriteReg(IER, 0x00);

	// special mode to set baud rate.
	WriteReg(LCR, LCR_BAUD_LATCH);

	// LSB for baud rate of 38.4K.
	WriteReg(0, 0x03);

	// MSB for baud rate of 38.4K.
	WriteReg(1, 0x00);

	// leave set-baud mode,
	// and set word length to 8 bits, no parity.
	WriteReg(LCR, LCR_EIGHT_BITS);

	// reset and enable FIFOs.
	WriteReg(FCR, FCR_FIFO_ENABLE | FCR_FIFO_CLEAR);

	// enable transmit and receive interrupts.
	WriteReg(IER, IER_TX_ENABLE | IER_RX_ENABLE);

	initlock(&uart_tx_lock, "uart");
}

// add a character to the output buffer and tell the
// UART to start sending if it isn't already.
// blocks if the output buffer is full.
// because it may block, it can't be called
// from interrupts; it's only suitable for use
// by write().
void uartputc(int c)
{
	acquire(&uart_tx_lock);

	if (panicked) {
		for (;;) ;
	}

	while (1) {
		if (uart_tx_w == uart_tx_r + UART_TX_BUF_SIZE) {
			// buffer is full.
			// wait for uartstart() to open up space in the buffer.
			sleep(&uart_tx_r, &uart_tx_lock);
		} else {
			uart_tx_buf[uart_tx_w % UART_TX_BUF_SIZE] = c;
			uart_tx_w += 1;
			uartstart();
			release(&uart_tx_lock);
			return;
		}
	}
}

// alternate version of uartputc() that doesn't 
// use interrupts, for use by kernel printf() and
// to echo characters. it spins waiting for the uart's
// output register to be empty.
void uartputc_sync(int c)
{
	push_off();

	if (panicked) {
		for (;;) ;
	}

	// wait for Transmit Holding Empty to be set in LSR.
	while ((ReadReg(LSR) & LSR_TX_IDLE) == 0) ;
	WriteReg(THR, c);

	pop_off();
}

// if the UART is idle, and a character is waiting
// in the transmit buffer, send it.
// caller must hold uart_tx_lock.
// called from both the top- and bottom-half.
void uartstart()
{
	while (1) {
		if (uart_tx_w == uart_tx_r) {
			// transmit buffer is empty.
			return;
		}

		if ((ReadReg(LSR) & LSR_TX_IDLE) == 0) {
			// the UART transmit holding register is full,
			// so we cannot give it another byte.
			// it will interrupt when it's ready for a new byte.
			return;
		}

		int c = uart_tx_buf[uart_tx_r % UART_TX_BUF_SIZE];
		uart_tx_r += 1;

		// maybe uartputc() is waiting for space in the buffer.
		wakeup(&uart_tx_r);

		WriteReg(THR, c);
	}
}

// read one input character from the UART.
// return -1 if none is waiting.
int uartgetc(void)
{
	if (ReadReg(LSR) & 0x01) {
		// input data is ready.
		return ReadReg(RHR);
	} else {
		return -1;
	}
}

// handle a uart interrupt, raised because input has
// arrived, or the uart is ready for more output, or
// both. called from trap.c.
void uartintr(void)
{
	// read and process incoming characters.
	while (1) {
		int c = uartgetc();
		if (c == -1)
			break;
		consoleintr(c);
	}

	// send buffered characters.
	acquire(&uart_tx_lock);
	uartstart();
	release(&uart_tx_lock);
}
